In 3GPP, Coordinated Multipoint Transmission (CoMP) is considered as a tool to improve the coverage of high data rates and cell-edge throughput and/or to increase system throughput. In CoMP transmission, two modes are defined, one is joint transmission (JT), and the other is coordinated scheduling/beamforming (CS/CB). In CoMP transmission, multiple transmission point devices (e.g., base station, Node B, or evolved Node B (eNB)) can transmit data simultaneously for one user equipment (UE). Assume that we have M transmission point devices and each device has K antenna ports, wherein the antenna port may in practice be implemented either as a single physical transmit antenna or as a combination of multiple physical antenna elements, then at time t and frequency f, the signal coming from the kth port of the mth transmission point device can be given by:rk,m(f,t)=exp(−j2π(f+Δfk,m)(t+τk,m)),k=0,1, . . . ,K−1  (1)wherein Δfk,m and τk,m are frequency offset and time offset of the kth port of the mth transmission point device relative to a reference port, respectively. The phase difference between the (k1,m1)th and (k2,m2)th ports isθk1,k2,m1,m2(f,t)=−j2π((Δfk1,m1−Δfk2,m2)t+(τk1,m1−τk2,m2)f+ϕk1,k2,m1,m2)  (2)wherein ϕk1,k2,m1,m2=Δfk1,m1τk1,m1−Δfk2,m2τk2,m2  (3)
From equation (2), we can see that the phase difference has close relationship with FAE (Δfk1,m1−Δfk2,m2) and TAE (τk1m1−τk2,m2), wherein the FAE and TAE are respectively frequency offset and time offset between antenna ports. WO 2008/097187 discloses a method for calibrating time misalignment (i.e., TAE) of signal transmitted from a transmitting unit to a receiving unit.
For CoMP transmission, the TAE includes subframe time offset, transmission delay and time offset between ports. The TAE is expected to be much larger than the TAE in a single cell, e.g., 65 ns. Even with 65 ns, according to equation (2), the maximum phase difference between lowest and highest sub-carriers is higher than 2.3 π for a system with 20M bandwidth. Such a time misalignment will typically result in increase of frequency selectivity, especially for a system with wide bandwidth. Without knowledge of the time misalignment at the transmitter, CoMP with joint precoding will seriously suffer, especially under the strategy of wideband precoding with one precoding matrix index (PMI) since frequency variations due to TAE do not match wideband precoding any more.
Besides TAE, as shown in equation (2), the FAE also has great impact on the precoding performance. In practice, frequency offset between the transmission point device and UE is conventionally estimated and compensated by UE to improve demodulation performance. FAE may not be a significant problem in a single-cell transmission with one remote radio unit (RRU) multiple-input multiple-output (MIMO) since the same oscillator is used for RF chains connecting to multiple transmit antennas. However, it is typically impossible in practice to use the same oscillator for multiple geometrically-separated transmission points, e.g. CoMP joint transmission (CoMP-JT). According to current 3GPP RAN4 specifications, the minimal requirement of frequency error is within ±0.05 ppm for a wide area base station. With this requirement, it is assumed that feedback delay is 5 ms and the frequency is 2.6 GHz. According to equation (2), the phase difference between the feedback phase and usage phase is 2.6π. This may cause CoMP transmission to fail.
In demodulation reference signal (DMRS)-based precoding technique, it is difficult to compensate the FAE and TAE at UE side. Since in DMRS-based precoding technique, the channel seen by UE is composite from multiple points, UE can not differentiate the signal from different antennas. Hence, FAE and TAE cannot be fully compensated. Whereas in cell-specific reference signal (CRS)-based or channel state information reference signal (CSI-RS)-based precoding technique, FAE and TAE can be estimated.